A jounce bumper (also called a bump stop, rebound bumper, end-of-travel bumper, strike-out bumper, suspension bumper, or compression bumper) is a shock-absorbing device ordinarily positioned on the top of vehicle suspensions. Jounce bumpers for use in motor vehicle suspension systems have long been used for cushioning the impact between two suspension system components, such as the axle and a portion of the frame, as well as for attenuating noise and vibration to increase the ride comfort of the passengers. Since displacement of the vehicle chassis causes displacements of the strut, the strut undergoes cycles of compression and extension in response to the displacement of the vehicle chassis. Provision must be made for protecting the strut assembly and the vehicle body from the jounce forces associated with severe irregularities in the road surface leading to extreme displacement of the suspension. For this reason, a jounce bumper is attached to the suspension system at a point where impact is likely to occur when the shock absorber fails to absorb the forces created by extraordinary driving conditions. Particularly, during jounce motions of the strut, the damper “bottoms out” and the jounce bumper moves into contact with the jounce bumper plate and compresses to dissipate energy resulting in cushioning the impact, reducing noise, reducing the sensation of impact to the passengers and reducing possible damage to the vehicle suspension system. Jounce bumpers are elongated, generally cylindrical or conical, members with or without convolutes, made of a compressible and elastomeric material that extends around the piston rod. As taught in U.S. Pat. No. 4,681,304, convoluted bumpers function by a progressive stacking of the convolutions to provide resistance to jounce forces.
Materials suitable for this application must be resilient, i.e. capable of withstanding shock without undue permanent deformation or rupture, and must have excellent flex life. Conventional jounce bumpers are formed of foamed polyurethane and vulcanized rubber. For example, jounce bumpers are often formed of microcellular polyurethane (MCU). A microcellular polyurethane jounce bumper is made by casting polyurethane precursors in a jounce bumper mold. Microcellular foam is obtained from the reaction of diisocyanate glycol with a blowing agent or with water which produces carbon dioxide gas for foaming. This technology is time-consuming since foaming requires prolonged times in the mold due to the slow release of carbon dioxide. While jounce bumpers made of foamed polyurethane have good ride characteristics, they are expensive to produce since they require an energy- and time-consuming technology due to the crosslinking.
With the aim of improving durability, inertness to automotive fluids, and resistance to tear propagation of the material used to form the jounce bumper, U.S. Pat. No. 5,192,057 discloses an elongated hollow body formed of an elastomer, preferably from a copolyetherester polymer. As disclosed therein, such pieces, including jounce bumpers having bellows shaped sections with a constant thickness profile, are manufactured by blow molding techniques. An alternative method for forming jounce bumpers, i.e. corrugated extrusion, is described in U.S. Published Patent Application No. 2008/0272529.
In a typical blow molding operation for manufacturing hollow plastic articles a parison of plastic material that has been produced by extrusion or injection molding and which is in a hot moldable condition is positioned between two halves of an open blow mold having a mold cavity of a shape appropriate to the required external shape of the article to be manufactured. The parison gradually moves and stretches under the influence of gravity. When the parison reaches the proper length, the mold halves are closed around it and pressurized air or other compressed gas is introduced in the interior of the parison to inflate it to the shape of mold or to expand it against the sides of the mold cavity. After a cooling period, the mold is opened and the final article is ejected.
In extrusion blow molding, the parison is produced by extruders. Extrusion blow molding is less expensive than foaming/casting but leads to less precise dimensions and leads also to limitations in the wall thickness of the part. The stiffness of a jounce bumper is directly related to its thickness. Thus, a small variation of thickness (either variation from article to article, along the longitudinal axis of a jounce bumper made from one shot, or along the radius of the convolute of a jounce bumper made in a single jounce bumper), for example 0.2 mm, will significantly change the stiffness of the jounce bumper and its energy absorption capacity and dampening performance.
Injection blow molding gives more precise dimensions than extrusion blow molding. In this technique, the parison is formed by injection molding, the inner core of the mold is removed and the parison is quickly inflated while being enclosed in two mold halves as in extrusion blow molding. The parison can be injection molded to have a non-constant cross-section resulting in a better wall thickness uniformity of the final part than from extrusion blow molding. Injection blow molding allows more precise details in the final blown structure but is more expensive than extrusion blow molding.
In general, it is desired to maximize the absorption of energy in a jounce bumper. The energy absorption behavior of a jounce bumper can be measured, for example, by measuring deformation versus applied force. Usually deformation is plotted on the X-axis (in mm), and applied load (force) is plotted on the Y-axis (in N). The area under the curve represents the energy absorbed by the jounce bumper according to the formuladisplacement×Force=energy.
Thermoplastic jounce bumpers made by any of the above-mentioned techniques can exhibit different responses depending on design, including specific configuration details, and materials of manufacture. There remains a need to improve the design of thermoplastic jounce bumpers so as to improve the force-displacement behavior, thereby increasing the energy absorbed.